1. Field of the invention
The present invention relates, in general, to a method for fabricating a multilayer electroluminescence device emitting a light of blue color and, more particularly, to a method which can prevent a peeling phenomenon of delaminating constituent plies in a luminescent layer by adopting SrS and CaS as materials for the constituent plies.
2. Description of the Prior Art
Until recently, there was extensively used a cathode ray tube (hereinafter referred to as "CRT") as an information display device. To date, as the information display device is strongly required to be less heavy, more solid and more flat, research and development for next generation information display devices has actively proceeded. As a result, an electroluminescence device, a liquid crystal display, a light emitting diode, and a plasma display panel are developed.
Among such information display devices, particularly, an electroluminescence device, an active solid display device in which hot electrons produced by high electric field may be interacted with a luminescent center to emit light, largely attracts scientific and commercial attention and its development is being watched with keen interest because it can have large display area and show superiority in luminance, color contrast and view angle.
Recently, in an electroluminescence device which is based on ZnS, respective luminances of yellow color and green color have been able to be obtained sufficiently to put the electroluminescence device into practice but neither red color nor blue color has been able to be obtained enough.
To accomplish multicoloration yet high luminance in an electroluminescence device, active research and study has been directed to novel luminescent basic substances, for example, a CaS and a SrS luminescent phosphors, instead of the conventional basic substance, ZnS.
Meanwhile, a white light EL device, a newly developed device, in which double luminescent centers are added into alkali earth luminescent phosphor has been and continues to be actively researched. Since it is proved that the white light EL device is able to control three primary colors with a filter as well as be utilized as a monocolor display device, it significantly attracts attention.
Now, in order to better understand the background of the present invention, a description will be made of a conventional multilayer electroluminescence device and a method for fabricating the same, along with its problems.
Referring initially to FIG. 1, there is shown a structure of a conventional multilayer electroluminescence device. As shown in this figure, the conventional multilayer electroluminescence device has a substrate 1 on which a lower electrode 2, a first insulation layer 3, a multiply luminescent layer 4, a second insulation layer 5 and an upper insulation layer 6 are in sequence deposited with the lower and the upper electrodes 2, 5 formed in respective predetermined patterns.
Referring now to FIG. 2, there is in detail shown the luminescent multilayer 4. As shown in the FIG. 2, a ZnS ply 7 and a SrS ply 8 doped with cesium for emission of a light of blue color are alternatively laminated with the uppermost layer and the lowest layer formed of ZnS, in the multiply luminescent layer 4. Accordingly, the uppermost ply and the lowest ply, both made of ZnS, come into contact with the first insulation layer 3 and the second insulation layer 5, respectively.
With regard to function of the multiply luminescent layer 4, the upper and lower plies of ZnS in the multiply luminescent layer 4 serve as buffer layers which not only prevent the first and second insulation layers 3, 5 from chemically reacting with the SrS ply 8, a fluorescent layer, but also improve crystallinity of the SrS ply 8. On the other hand, the intermediate plies of ZnS 7 in the multiply luminescence layer play a role of acceleration layers in which excited electrons are accelerated. The intermediate plies of SrS 8 emit light by means of the accelerated electrons.
Following is a description of a fabrication method for the conventional multilayer electroluminescence device.
First, on a substrate 1, for example, a glass substrate, there is deposited indium tin oxide (hereinafter referred to as "ITO") at a thickness of about 2,000 Angstrom which is then subjected to photolithography, to form a transparent lower electrode 2 of ITO having a predetermined pattern.
Subsequently, using a sputtering process, a material selected from a group consisting of, for example, Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, Ta.sub.2 O.sub.5, SiO.sub.2, SiON, SrTiO.sub.3, BaTiO.sub.3, PLZT and PbTiO.sub.3, is deposited at a thickness of about 3,000 Angstrom on the substrate 1 provided with the lower electrode 2, so as to form a first insulation layer 3.
Thereafter, using an electron beam evaporation or sputtering process, a ZnS ply 7 is deposited at a thickness of about 1,000 Angstrom at 220.degree. C. on the first insulation layer 3, followed by deposition of a SrS ply 8 doped with cesium in a thickness of 1,500 Angstrom at 500.degree. C. on the ZnS ply 7. The cesium doping emits light of blue color. Until the 5 to 8 SrS plies are formed in the multiply luminescent layer 4, other ZnS plies 7 and other SrS plies 8 are alternatively laminated in the just-mentioned manner. As previously described, the multiply luminescent layer 4 has an lowest ply and an uppermost ply which both are formed of ZnS.
Next, a second insulation layer 5 is deposited in a thickness of about 3,000 Angstrom on the luminescent multilayer 4. As a result, the lowest ply and the uppermost ply of the multiply luminescent layer come into contact with the first insulation layer 3 and the second insulation layer 5, respectively.
Finally, a metal, for example, aluminum is deposited at a thickness of about 2,000 Angstrom, which is then subjected to photolithography, to form an upper electrode having a predetermined pattern.
In such conventional electroluminescence device, application of an alternating current voltage of about 150 V to both the upper and lower electrodes produces high electric fields at the interface between the first insulation film 3 and the luminescent multilayer 4 and at the interface between the second insulation layer 5 and the luminescent multilayer 4. By virtue of these high electric fields produced at the interfaces, electrons which are in an interface state between the first insulation layer 3 and the luminescent multilayer 4 and in an interface state between the second insulation layer 5 and the luminescent multilayer 4 are accelerated and thus transformed into so-called hot electrons with tunneling to a conduction band of the ZnS ply 7 of the luminescent multilayer.
While a portion of the hot electrons impact upon a luminescent center, for example Mn.sup.2+ doped in ZnS which is a base substance of the multiply luminescent layer 4, to excite the luminescent center, a portion of the hot electrons ionizes the base substance, coupling with holes. As a result, electron-hole pairs are produced.
As the hot electrons travel in the luminescent layer 4, they further impact upon cesium, the dopant of the SrS ply 8, to emit light. For this luminescent mechanism, the outermost electrons of the cesium are excited to the conduction band by absorbing energy upon impact and then falls into a valence band. In the meanwhile, a light corresponding to the same energy as difference between the conduction band and the valence band is emitted from the luminescent layer 4.
The reason why a multiply luminescent layer is employed in an electroluminescence device is that the intensity of a light emitted from trailing edge is more increased, allowing more bright luminance.
Respective plies of the multiply luminescence layer 4 are made of Periodic table II-VI group compounds, that is, alkali earth sulfides and have large band gap. Practically representative substances for the multiply luminescence layer includes SrS and ZnS.
Lattice constants of SrS and ZnS are 1.13 Angstrom and 0.74 Angstrom, respectively. The difference of the lattice constant between SrS and ZnS reaches even 35%, which is significantly larger than the maximum allowable difference of the lattice constant required to obtain a matched interface, 16%. Therefore, lamination of the two plies showing a large difference of lattice constant, the SrS ply 8 and the ZnS ply 7 leads to a mismatched interface therebetween, as shown in FIG. 3.
At the mismatched interface between the SrS ply 8 and the ZnS play 7 laminated in the luminescent layer of the conventional luminescence device, stress causes a peeling phenomenon that the SrS ply 8 and the ZnS play 7 are delaminated, ultimately breaking the multilayer electroluminescence device.
In addition, the ZnS ply 7 is deposited at 220.degree. C., whereas the SrS ply 8 is at 500.degree. C. Accordingly, to effect alternative deposition of the ZnS ply 7 and the SrS ply 8 in the conventional multilayer luminescence device, the deposition temperature must be changed alternatively, adding thermal stress into the two plies. Wherefore, a significant disadvantage of the prior art luminescence device is that this thermal stress is a main factor aggravating the peeling phenomenon.